Generally, in the bush cutters, a drive shaft passed through a pipe-shaped operating rod is driven by a drive source, provided on a rear end portion of the operating rod, to rotate a cutter blade provided on a front end portion of the operating rod. Among the conventionally-known types of bush cutters are the shoulder-hanging type and backpack type. In the case of the shoulder-hanging type, a human operator slings the bush cutter over one of the shoulders in such a manner that the operating rod extends from near the operator's waist obliquely forward and downward, and cuts grass by swinging the operating rod up and down, back and forth and left and right while grabbing an operating hand provided on an intermediate portion of the operating rod. One example of such a shoulder-hanging type bush cutter is disclosed in Japanese Patent Application Laid-open Publication No. 2005-237265 (hereinafter referred to as “the patent literature”).
The bush cutter disclosed in the patent literature, a gearbox is provided on the front end portion of the pipe-shaped operating rod, and a bevel gear unit is provided within the gearbox. Driving power of the drive source is transmitted from a drive shaft to an output shaft via driving bevel gear and driven gear, so that the cutter blade provided on the front end portion of the output shaft can be rotated by the driving power. The gearbox has one end portion opening toward the cutter blade, and the output shaft and bevel gear unit are built into the gearbox through this open end portion. The output shaft is supported in the gearbox via a pair of upper and lower bearings, and the cutter blade is attached, via a fastening member, to a lower end portion of the output shaft projecting downward through the one end portion. The lower bearing adjacent to the cutter blade is fastened to the gearbox via a retaining ring. A labyrinth mechanism is provided between the surface of the open end portion and the fastening member, and this labyrinth mechanism serves to prevent foreign matters, such as cut grass and pebbles, from entering the gearbox during bush cutting work using the bush cutter.
The labyrinth mechanism comprises an extremely narrow maze-like gap. If the bush cutter is used for a long time, foreign matters would undesirably bite into the labyrinth mechanism and wind around the surface of the open end portion, so that the surface of the open end portion tends to be worn. As the wear of the surface of the open end portion progresses, the retaining ring fastening the lower bearing, located adjacent to the cutter blade, to the gearbox may be undesirably detached downward from the gearbox. As a consequence, the lower bearing and the output shaft may be detached downward from the gearbox.
To avoid such unwanted detachment of the lower bearing and output shaft, an extra or backup retaining ring is attached to an upper end portion of the output shaft remote from the one end portion having the cutter blade provided thereon. The backup retaining ring is held in contact with the upper surface of an inner race of the upper bearing, and thus, even when the above-mentioned retaining ring fastening the lower bearing to the gear box (i.e., main retaining ring) has been detached from the gearbox, the backup retaining ring can still retain the output shaft. Because the output shaft is prevented from moving downward by the backup retaining ring, it will not be detached downward from the gearbox. Because the driving power of the drive source can continue to be transmitted from the bevel gear unit to the output shaft even after the detachment of the main retaining ring, the cutter blade can continue to rotate. However, to permit timely and prompt maintenance and checkup of the bush cutter, it is desirable that a human operator be allowed to recognize a state in which the main retaining ring fastening the lower bearing to the gearbox is detached from the gearbox.